1. Field of the Invention
The present invention relates to a control device and a method for operating a self-igniting internal combustion engine.
2. Description of Related Art
Combustion methods used in gasoline engines, often called HCCI methods (homogenous charge compression ignition) or CAI methods (controlled auto ignition) in the literature, are characterized in that a fuel supplied to an internal combustion engine is not externally ignited, but rather is ignited on the basis of a compression and/or a heating of the fuel. In comparison with conventional external ignition methods, these methods are distinguished by economical fuel consumption and low raw pollutant emission. The latter method also has the additional advantage that a relatively expensive exhaust gas aftertreatment system, such as for example a NOx storage catalytic converter, can be done without.
Frequently, in a self-igniting combustion method, before the beginning of the actual injection process a small quantity of fuel is introduced into a cylinder of the internal combustion engine. This is known as a pre-injection, via which the combustion chamber is prepared for the main combustion that follows in the actual operating cycle. The fuel introduced into the cylinder as a pre-injection can be ignited during an intermediate compression. The resulting release of energy causes heating of the combustion chamber, thus supporting the combustion of the main injected fuel quantity in a CAI method.
The quantity of fuel introduced into the internal combustion engine as a pre-injection is also sometimes referred to as a null quantity. By null quantity is meant the smallest possible quantity of a fuel that still results in effective energy release in the following intermediate compression. Null quantity may also refer to a control duration of an injector.
However, in a self-igniting internal combustion engine, and in a corresponding combustion method, the direct trigger in the form of the external ignition for the initiation of the combustion is not present. The combustion position must therefore be carefully adjusted using control quantities of the internal combustion engine. The combustion position is frequently indicated by a crank angle, and designates a specific energy conversion, for example the combustion center MFB50 (mass fraction burned 50%).
Here, the division of the overall injected quantity into a pre-injection and a main injection can in particular have large effects on the combustion position. Even very small deviations of the ratio of the pre-injection to the main injection can have drastic effects on the combustion position. If a combustion takes place too late, it becomes unstable, and a permanent failure may result. If, on the other hand, the combustion takes place too early, unacceptably loud noise results, and the tendency towards engine knock increases. It is therefore desirable to provide a possibility by which a pre-injection can be carried out in such a way that an acceptable combustion position is ensured.